The invention relates to a vehicle drive device with a hydrostatic mechanical power splitting transmission between a drive prime mover and a driven mechanism--for instance a vehicle axle drive train--comprising a planetary differential transmission including at least two groups of planet wheels meshing with two sun wheels and at least four drive shafts, a web, and at least one annulus, two of these shaft corresponding to two main shafts of the transmission, namely an input shaft and an output shaft, further at least two variable displacement hydrostatic machines of the swash plate or bent axle type adapted to operate as a pump and as a motor, and in at least one operational range are connected to its respective shaft of the planetary differential transmission, such respective shaft being a shaft different from the main shafts, a transition from a first operational range to a second one taking place when the speed of a first given one of the hydrostatic machines drops to substantially zero, the second of the hydraulic machines then being switched over by means of clutches from the said output shaft to its own shaft through at least one of said sun wheels.
A drive device with these features has been proposed in U.S. Pat. No. 4,313,351. In the case of vehicle drives designed on this basis use was made of a hydromechanical power splitting transmission with two variable displacement hydrostatic machines able to be operated as motors or pumps, each one of which was able to transmit a rated power of about 2/3 from the power of the drive prime mover. This drive arrangement makes it possible to attain an approximately 5-fold increase in torque between the drive prime mover and the axle drive train.
This known design makes possible operation in a broad overdrive range but only when the speed of the drive prime mover and of the output of the drive is low and at the sacrifice of an increase in reactive power taking place as from a speed ratio of N.sub.output :N.sub.input greater than approximately 1.5, that is to say with a decrease in efficiency. The latter feature is more especially a disadvantage in the case of use of the drive device in a car owing to the wide range of speed of rotation of the drive engine used therein.
In some applications the above-mentioned increase in torque of a factor of 5 is not sufficient in order to achieve a sufficient overall conversion range. In the case of very heavy tracklaying or wheeled vehicles a sufficient overall conversion range would only be on the basis of an increase in torque by a factor of 10 at full engine power. This problem has been able to be remedied so far by the use, for instance, of high power hydrostatic machines. SUch larger hydrostatic machines would however not only increase the overall size of the power splitting transmission, but also would involve redesigning hydraulic system parts such as power lines, control means, valves and the like so as to be suitable for a higher flow rate and an increased power, something that would obviously lead to a substantial increase in the initial price of such a power splitting transmission. Furthermore there is usually not sufficient space for such redesign to be effected.